Markos Zaharioudakis

Answering complex SQL queries using automatic summary tables

We investigate the problem of using materialized views to answer SQL queries. We focus on modern decision-support queries, which involve joins, arithmetic operations and other (possibly user-defined) functions, aggregation (often along multiple dimensions), and nested subqueries. Given the complexity of such queries, the vast amounts of data upon which they operate, and the requirement for interactive response times, the use of materialized views (MVs) of similar complexity is often mandatory for acceptable performance. We present a novel algorithm that is able to rewrite a user query so that it will access one or more of the available MVs instead of the base tables. The algorithm extends prior work by addressing the new sources of complexity mentioned above, that is, complex expressions, multidimensional aggregation, and nested subqueries. It does so by relying on a graphical representation of queries and a bottom-up, pair-wise matching of nodes from the query and MV graphs. This approach offers great modularity and extensibility, allowing for the rewriting of a large class of queries.

Fine-grained sharing in a page server OODBMS

For reasons of simplicity and communication efficiency, a number of existing object-oriented database management systems are based on page server architectures; data pages are their minimum unit of transfer and client caching. Despite their efficiency, page servers are often criticized as being too restrictive when it comes to concurrency, as existing systems use pages as the minimum locking unit as well. In this paper we show how to support object-level locking in a page server context. Several approaches are described, including an adaptive granularity approach that uses page-level locking for most pages but switches to object-level locking when finer-grained sharing is demanded. We study the performance of these approaches, comparing them to both a pure page server and a pure object server. For the range of workloads that we have examined, our results indicate that a page server is clearly preferable to an object server. Moreover, the adaptive page server is shown to provide very good performance, generally outperforming the pure page server, the pure object server, and the other alternatives as well.

Adaptive, fine-grained sharing in a client-server OODBMS: a callback-based approach

For reasons of simplicity and communication efficiency, a number of existing object-oriented database management systems are based on page server architectures; data pages are their minimum unit of transfer and client caching. Despite their efficiency, page servers are often criticized as being too retrictive when it comes to concurrency, as existing systems use pages as the minimum locking unit as well. In this paper we show how to support object-level locking in a page-server context. Several approaches are described, including an adaptive granularity approach that uses page-level locking for most pages but switches to object-level locking when finer-grained sharing is demanded. Each of the approaches is based on extending the idea of callback locking. We study the performance of these approaches, comparing them to both a pure page server and a pure object server. For the range of workload that we have examined, our results indicate that the adaptive page server provides very good performance, usually outperforming the pure page server and the other page-server variants as well. In addition, the adaptive page server is often preferable to the pure object server; our results provides insight into when each approach is likely to perform better.

Highly concurrent cache consistency for indices in client-server database systems

In this paper, we present four approaches to providing highly concurrent B+-tree indices in the context of a data-shipping, client-server OODBMS architecture. The first performs all index operations at the server, while the other approaches support varying degrees of client caching and usage of index pages. We have implemented the four approaches, as well as the 2PL approach, in the context of the SHORE OODB system at Wisconsin, and we present experimental results from a performance study based on running SHORE on an IBM SP2 multicomputer. Our results emphasize the need for non-2PL approaches and demonstrate the tradeoffs between 2PL, no-caching, and the three caching alternatives.

Hierarchical, adaptive cache consistency in a page server OODBMS

Due to its simplicity and communication efficiency many client-server object-oriented database systems are based on the basic page server architecture-pages serve as their smallest unit of data transfer, client caching, and concurrency control. In an earlier paper, we showed how to extend this architecture to permit object-level callback locking, and we showed through simulations that significant performance gains can be expected. In the current paper we report on our experiences from implementing this approach in the context of the SHORE system, which supports a generalized peer-servers architecture. In addition to discussing some of the stickier implementation details, we also explain how our callback algorithm was extended to support the hierarchical locking approach used in SHORE. Finally, we present performance measurements obtained by running SHORE on an IBM SP2 machine.